Ink jet printers having one or more ink jet heads for projecting drops of ink onto paper or other printing medium to generate graphic images and text have become increasingly popular. To form color images, multiple ink jet printing heads are used, with each head being supplied with ink of a different color from an associated ink cartridge.
In a common arrangement, the print medium is attached to a rotating drum, with the ink jet heads being mounted on a travelling carriage that traverses the drum axially. As the heads scan spiral paths over the medium, ink from the ink cartridges is delivered to the ink jet heads. Ink drops developed within the heads are projected from a minute orifice to form an image on the medium. A suitable control system synchronizes the generation of ink drops with the rotating drum.
Such printers commonly employ replaceable ink cartridges. One such known cartridge, designated the Maco cartridge, is produced by Matsushita Electronic Components Co., Ltd. of Japan. The Maco cartridge has an internal ink container which includes a collapsible ink bag and an ink bag support. The ink bag and support are clamped together by a mechanical seal. The assembled ink bag, ink bag support and mechanical sealing components are positioned within a plastic housing. An ink flow passageway is provided for delivering ink from the ink container to an ink jet head of the printer. Also, an air flow passageway is provided through which pressurized air is delivered by the printer to the portion of the interior of the housing which is outside of the ink container. This pressurized air applies pressure to the ink bag and urges ink from the ink bag through the ink flow passageway.
In addition, the Maco cartridge includes a pair of spaced apart elongated stainless steel probes which extend from the exterior of a cap portion of the housing, through the ink bag support, and into the interior of the ink bag. The tips of the probes are thus exposed to ink contained within the bag. More specifically, in the Maco cartridge, the probes pass through a wedge-shaped projection extending from the ink bag support into the ink container, the wedge being tapered along its free edge. The tips of the probes extend beyond this free edge and into ink within the bag. In operation, the ink jet printer applies a voltage to the probes. The printer monitors the electrical resistance in a conducting path through the ink between the tips of the probes. As ink is used from the bag, the bag collapses. This changes the resistance in the conductive path between the probes. In theory, with the Maco construction, by monitoring this resistance, the volume of ink in the container is known. In addition, the ink jet printer is designed to automatically shut off when the resistance reaches a predetermined level. This is intended to prevent the ink jet head from being clogged with an air bubble which may form in the ink jet head if the printer continues to operate after the cartridge is empty.
The Maco ink level sensor suffers from a number of disadvantages. In particular, this device produces inconsistent resistance determinations for the same amount of ink within the ink bag. At times when an ink bag is substantially empty, the probe resistance readings may indicate that significant amounts of ink remain in the bag. This subjects the ink jet heads to clogging by air bubbles generated when the ink jet printer is operated with a substantially empty cartridge. In contrast, at other times, when substantial amounts of ink remain in the ink bag, the probe readings may correspond to an empty ink bag. Under these circumstances, changing of the ink cartridge results in wasted ink.
Therefore, a need exists for an ink cartridge which overcomes these and other disadvantages of the prior art.